1. Field of the Invention
The present invention relates to apparatuses and methods for subband synthesis filtering and, in particular, to the apparatuses and methods used in audio decoders.
2. Description of the Prior Art
The Motion Pictures Experts Group (MPEG) audio signal specification provides standard encoding/decoding algorithms for audio signals. The algorithms in the MPEG specification can significantly reduce bandwidth demands for transmitting audio data and can also provide audio signals with little distortions. Currently, the encoding/decoding algorithms in the MPEG audio signal specification are mainly divided as three layers: Layer I, Layer II, and Layer III.
Please refer to FIG. 1, which illustrates the flowchart of decoding an audio frame according to the MPEG-1 Layer III (MP3) algorithm. In Step S11, a header of the audio frame is unpacked, and the side information of the audio frame is decoded. Next, in step S12, audio data compressed with Huffman encoding method in the audio frame is decoded, and re-quantization is performed on the frequency components generated after Huffman decoding. In Step S13, joint stereo processing is performed on the frequency components according to audio modes recorded in the side information. In Step S14, alias reduction is performed on the frequency components. In Step S15, inverse modified discrete cosine transform (IMDCT) is performed on the frequency components. In Step S16, frequency inversion is performed on the sample signals generated after IMDCT. In Step S17, subband synthesis filtering is performed to synthesize pulse code modulation (PCM) signals based on the sample signals. At last, step S18 is executed to output the PCM signals, so as to complete this decoding procedure of the audio frame.
In the MP3 standards, there are 576 sample signals generated after IMDCT in step S15. The 576 sample signals include sample signals of 32 subbands; each subband respectively corresponds to 18 sample signals. Assume the 32 subbands are respectively numbered as the 0th subband, the 1st subband . . . , and the 31st subband, and the 18 sample signals in one subband are respectively numbered as the 0th sample signal, the 1st sample signal . . . , and the 17th sample signal. In the frequency inversion in step S16, the 1st, 3rd . . . , and 17th sample signals in the 1st, 3rd, . . . , and 31st subbands are respectively multiplied by negative one, that is, multiplying the odd numbered sample signals in the odd numbered subbands by negative one.
Please refer to FIG. 2, which illustrates the flowchart of the process of the frequency inversion in prior arts. This procedure starts at step S20. In Step S21, sample signals corresponding to a certain subband among the 32 subbands are accessed. In Step S22, this subband is being judged whether it is an odd subband among the 32 subbands. If the judging result of step S22 is YES, step S23 is performed; otherwise, step S27 is then performed. In Step S23, one of the 18 sample signals of this subband is accessed. In Step S24, the sample signal accessed in step S23 is being judged whether it is an odd sample signal among the 18 sample signals of this subband. If the judging result of step S24 is YES, step S25 is performed; otherwise, step S26 is then performed. In Step S25, the sample signal accessed in step S23 is multiplied by negative one. In Step S26, it is judged whether all the 18 sample signals in this subband have been accessed. If the judging result of step S26 is YES, step S27 is performed; otherwise, step S23 is performed again. In Step S27, it is judged whether all the 32 subbands have been accessed. If the judging result of step S27 is YES, step S28 is performed to end this procedure; otherwise, step S21 is performed again.
As shown in FIG. 2, the frequency inversion procedure in prior arts includes numerous accessing and judging steps. These steps take up a lot of processing time and accordingly decrease the efficiency of decoding the MP3 audio frames.
Subband synthesis filtering, the step next to frequency inversion, is generating PCM signals based on subband sample signals after frequency inversion. There have been prior arts for converting 32 sample signals into 32 converted vectors by 32-point discrete cosine transform (DCT).
Please refer to FIG. 3, which illustrates the flowchart of a subband synthesis filtering procedure using 32-point DCT in the prior art. In this procedure, the 576 sample signals generated after frequency inversion are divided into 18 sets of signals. Each set of the signals respectively includes 32 sample signals; each of the 32 sample signals corresponds to a respective subband. The 18 sets of signals are sequentially processed. In Step S31, the 32 sample signals being processed are inputted into the procedure or apparatus of subband synthesis filtering. Next, in step S32, the 32 sample signals are converted into 32 converted vectors. In Step S33, 32 PCM signals are generated based on the 32 converted vectors.